equilibria involving ions acids and bases
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Equilibria involving ions: acids and bases. AH Unit 2(b)(iii). Key question. What is are acids and bases?. Arrhenius definition. An acid is a substance that when added to water increases the concentration of H + (aq) ions . HA + (aq)  H + (aq) + A - (aq)

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Presentation Transcript
key question
Key question
  • What is are acids and bases?
arrhenius definition
Arrhenius definition
  • An acid is a substance that when added to water increases the concentration of H+(aq) ions.

HA + (aq)  H+(aq) + A-(aq)

  • A base is a substance that when added to water increases the concentration of OH-(aq) ions.

BOH + (aq)  B+(aq) + OH-(aq)

key question1
Key question
  • What are the limitations of these definitions?
bronsted lowry definitions
Bronsted-Lowry definitions
  • An acid is a proton donor.

HA  H++ A-

HA + H2O  H3O++ A-

  • A base is a proton acceptor.

B + H+ BH+

B + H3O+  BH+ + H2O

ionisation of water
Ionisation of water
  • Water is amphoteric.
  • Can you write an expression for the equilibrium constant?
ionic product of water
Ionic product of water
  • Kw has a value of 1 x 10-14 at 25ºC.
  • Known as the ionic product of water.
  • Value varies with temperature.
dissociation of acids
Dissociation of acids

This is a measure of the strength of an acid

- the larger the value of Ka, the stronger the acid.

dissociation of bases
Dissociation of bases

This is a measure of the strength of a base

- the larger the value of Ka, the weaker the base.

indicators1
Indicators
  • Are weak acids
slide20
HIn and In- have different colours
  • Their ratio is dependant on [H3O+]
  • The colour of an indicator in any given solution therefore depends on the ratio, which in turn is determined by pH
slide21
The theoretical point at which the colour change occurs is when [HIn] = [In-]
  • Therefore the colour change occurs when KIn = [H3O+]
  • pKIn = pH
slide22
In practice, the colour change is not visible when [HIn] = [In-]
  • Instead, they must differ by a factor of 10
    • i.e. when [H+] = KIn± 10
    • OR when pH = pKIn ± 1
choice of indicator
Choice of indicator
  • Colour change must occur as close to the equivalence point as possible.
  • Equivalence point – the point at which all of the acid has been exactly “neutralised” by all of the alkali.
  • Does this always occur at pH 7?
slide26
The colour chance must occur in the region of rapid pH change.
  • This means that the addition of half a drop of acid/base will cause a colour change.
  • The choice of indicator must therefore be made with reference to titration curves.
buffer solutions
Buffer solutions
  • Is a solution where the pH remains approximately constant when small amounts of acid or bases are added.
  • Common examples:
    • blood
    • sea water
acid buffers
Acid buffers
  • Consists of a weak acid with one of its salts (of a strong alkali)
    • e.g. ethanoic acid + sodium ethanoate
  • The acid is partially dissociated and equilibrium with its ions.
  • The salt is fully ionised.
slide36
Addition of alkali:

Supplies H3O+(aq) ions if any removed in reacting with an added base.

  • Addition of acid:

CH3COONa(s) → Na+(aq) + CH3OO-(aq)

The conjugate base removes any added H+(aq)

ph of buffer solutions
pH of buffer solutions

Because the by diluting a buffer the concentration of acid and salt will decrease in proportion, dilution will not affect the pH of a buffer solution.

basic buffers
Basic buffers
  • Consist of a weak base with one of its salts (of a strong acid).
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